Method for Continuous Casting of a Metal with Improved Mechanical Strength and Product Obtained by Said Method

ABSTRACT

The invention concerns a method for continuous casting of a metal in the form of a hollow jet into a nozzle arranged between a pouring ladle or a tundish and a continuous casting ingot mold, said nozzle including in its upper part a dispensing member capable of deflecting at least part of the liquid metal reaching the nozzle inlet towards an inner wall of the nozzle before it penetrates into the ingot mold. Said method includes injecting into an inner volume of the hollow jet finely-divided solid material, characterized in that the finely-divided solid material comprises technical ceramic nanoparticles, of characteristic size less than 200 nm and preferably less than 100 nm.

FIELD OF THE INVENTION

The present invention relates to a new method for the continuous casting of a molten metal, in particular steel, that allows to obtain an intermediate product such as a slab, billet, wire, etc. before subsequent thermomechanical treatment such as lamination, continuous annealing, etc., such that its chemical composition is modified by the addition of elements in order to give it greater mechanical strength.

The following description makes more specific reference to the continuous casting of steel. However, this choice is only an example and does not entail any limitation of the invention.

The invention also relates to the product with improved mechanical features obtained by the method.

STATE OF THE ART

The technique of the continuous casting of steel is well known. It essentially consists in feeding molten steel from a ladle or from a tundish into a cooled copper or copper-alloy mould called “continuous casting ingot mould”, the latter being open at its bottom end, and in extracting from this opening an ingot in the form of a partly solidified continuous sheet.

In general, the molten steel is fed into the ingot mould by means of at least one nozzle, i.e. a generally tubular element positioned between the tundish and the ingot mould. The bottom end of the nozzle is usually provided with one or two outlet apertures located on the axis of the nozzle or on the sides, and comes out below the level that is free of molten steel present in the ingot mould.

Developments of the nozzles are also known that are intended to achieve improved cooling of the too-hot molten steel coming from the tundish. The aim is to obtain steel the form of a paste upon its entry into the ingot mould. These nozzles may in particular comprise a heat exchanger with a water-cooled copper tube or even a deflector or a dome. The latter has the purpose of forcing the overheated steel to trickle down in a thin layer along the walls of the nozzle, which allows to significantly increase the area of thermal exchange. The cooling of the conduit ensures the removal of the excess heat from the steel and causes the appearance of a solid fraction which turns the steel into a paste upon its entry into the ingot mould. The introduction of a protective gas under pressure, for example argon, in the conduit causes an overload that prevents any air flow by the molten steel, which would lead to its oxidisation or to the formation of alumina and the clogging of the nozzle. This technique described in patent EP-B-269-180 is called casting with a hollow jet or by means of a HJN or hollow jet nozzle.

Another development, described in patent EP-B-605 379, relates to the injection into the hollow jet of some quantity of finely divided metal material by using a non-oxidising gas as a vector at a slightly higher pressure relative to atmospheric pressure in order to prevent any entry of air. Depending on the case, the aim is to obtain refinement of the solidification structure by creating new solidification seeds or a modification of the basic chemical composition of the steel.

A continuous casting nozzle with a rotating jet is also known, as described in patent BE-A-101 20 37, and composed of a vertical conduit with a distribution device or dome in its upper part, whose function is also to divert the metal entering the nozzle towards the internal surface of said conduit and which comprises three arms arranged in a star pattern relative to the nozzle axis and canted relative to the horizontal. These arms are configured so as to impart a helicoidal rotary motion along the inner wall to the molten steel. The molten steel then comes out through two side outlets in the nozzle at a speed that is significantly lower than that obtained with a conventional nozzle with the same flow, which improves the quality of the ingots extracted (less inclusions and less gas bubbles).

The continuous casting of steel-based products with a mixed chemical or bi-component composition has also aroused great interest in a large number of specific applications, both for long and flat products (for example reduction of the silicon level at the surface of the slabs, in order to improve the suitability of laminated products to galvanisation; modification of the carbon content at the surface of peritectic steels to improve their casting flow; casting of products whose mechanical properties vary along their thicknesses, such as for instance great strength at the surfaces and high ductility in the cores, etc.). The term bi-component refers to products with a chemical composition of steel that varies depending on its position in the product studied, for example varying in the skin compared with the core. To meet this requirement, the Applicant proposed in international patent application WO-A-02/30598 a continuous casting nozzle comprising a distribution device with a dome in its top part, designed to separate the molten steel into two streams, an inner stream and an outer stream, in two physically well-separated zones. A means for injecting a gas, liquid or finely divided solid material (a powder with a particle size typically greater than 100 microns) under the dome into the inner zone allows the formation of a steel with a chemical composition that is different to that of the basic steel, cast in the outer zone.

In addition, it is known that traditional thermomechanical treatments aimed at improving the mechanical features of a steel, for example by its microstructure (martensite, bainite, etc.) or by endogenous precipitation, have the drawback that the structure of the steel finally obtained may be adversely affected by thermal post-treatment of the product (for example welding, galvanisation, etc.). It would therefore be desirable, at least in some cases, to be able to cast directly a product with a structure, and hence mechanical properties, that are stable throughout any subsequent treatment that the product might undergo.

Aims of the Invention

The present invention aims to provide a solution that allows to overcome the drawbacks of the state of the art.

The present invention aims in particular to provide a method of continuous casting that allows to produce slabs or billets of a modified chemical composition adapted to give the steel greater mechanical strength before lamination.

The invention notably aims to obtain a steel of homogeneous chemical composition and/or stabilised structure relative to a lamination process and/or thermomechanical treatment subsequent to casting.

One particular aim of the present invention is to exploit the hollow-jet technique in order to inject finely divided ceramic particles through the continuous casting nozzle.

Main Characteristic Elements of the Invention

A first aim of the present invention relates to a method for the continuous casting of a metal, in the form of a hollow jet in a nozzle positioned between a ladle or a tundish and a continuous casting ingot mould, said nozzle comprising in its upper part a distribution device capable of diverting at least part of the molten metal arriving at the inlet of the nozzle towards an inner wall in the nozzle before it enters the ingot mould, said method comprising the injection in an internal volume of the hollow jet of finely divided solid material, characterised in that the finely divided solid material comprises nanoparticles of technical ceramic, of a characteristic size lower than 200 nm, and preferably lower than 100 nm.

Advantageously, the nanoparticles of technical ceramic comprise nanoparticles of oxides, nitrides, carbides, borides, silicides and/or compounds thereof.

The oxides are preferably Al₂O₃, TiO₂, SiO₂, MgO, ZrO₂ or Y₂O₃.

As a further advantage, the size of the nanoparticles is between 10 and 100 nm.

Still according to the invention, the quantity of nanoparticles incorporated into the molten metal is lower than or equal to 5%, and preferably between 0.1 and 1% by weight of cast metal.

According to a preferred embodiment of the invention, the ceramic nanoparticles injected into the internal volume of the hollow jet of the nozzle are in suspension in a non-oxidising gas, preferably argon, said gas being at slightly higher pressure relative to atmospheric pressure and at most equal to the static pressure of the cast metal upon its entry into the ingot mould.

According to another preferred embodiment of the invention, the ceramic nanoparticles are injected into the internal volume of the hollow jet of the nozzle by means of a mechanical conveyance device such as a worm screw.

As a particular advantage, the nanoparticles are conglomerated prior to their injection into the nozzle into microparticles of a size essentiallyy between 10 and 1,000 microns, and preferably between 100 and 200 microns.

Still advantageously, prior to their injection into the nozzle, the nanoparticles are conglomerated into a metal matrix made of the same metal or of a different metal to the cast metal.

The cast metal is preferably molten steel and the metal matrix is an iron matrix or the metal matrix comprises a alloy metal other than iron.

As a further advantage, the conglomeration of the nanoparticles is obtained by mixing ceramic nanoparticles with micrometric iron particles, i.e. particles of a size greater than 10 microns, and preferably less than 20 microns.

According to a first preferred method, said mixture is produced by a pre-mix in a slurry, followed by drying, crushing, isostatic pressing and further crushing.

According to a second preferred method, said mixture is produced by high-energy tapping of the type “mechanical alloying” so as to incorporate the ceramics into the iron matrix.

According to a first advantageous embodiment, the hollow-jet nozzle used is of the type rotating jet, i.e. it comprises a vertical conduit having a distribution device with a dome in its upper part, whose function is to divert the molten metal entering the nozzle towards the internal surface of said conduit and which comprises a series of arms arranged symmetrically in a star pattern relative to the axis of the nozzle and canted relative to the horizontal, said arms being arranged to impart a helicoidal rotary motion to the molten steel along the inner wall of the nozzle.

According to another advantageous embodiment, the hollow-jet nozzle used comprises a distribution device with a dome in its upper part designed to separate the molten metal into two streams, an inner stream and an outer stream, in two physically well-separated zones, the injection of ceramic nanoparticles under the dome in the inner zone allowing the formation of a metal with a different chemical composition to that of the basic metal, cast in the outer zone.

Alternatively, the injection of ceramic nanoparticles may be carried out in the outer zone of the nozzle.

A second aim of the present invention relates to a metal, preferably steel, with high mechanical strength and taking the form after casting of an ingot in a continuous sheet upon its exit from a continuous casting ingot mould, specifically obtained by means of the above-described method and comprising less than one percent by weight of technical ceramic homogeneously distributed in at least one part of the ingot.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The idea on which the invention is based is to develop a steel hardened by a fine dispersion of ceramic particles that give the steel stable properties that do not deteriorate because of subsequent thermal treatment(s).

By way of an example, the case of the continuous casting of steel will be considered.

It is therefore proposed to cast a standard basic steel to which is added, as required, a quantity of particles needed to obtain the strength properties desired. As an advantage, the addition of particles to the molten metal is carried out directly at the level of the continuous casting nozzle since the latter, in the embodiments generally used and described above, generally comprises a means for inserting alloy elements or oxides in at least one fraction of the molten metal passing through the nozzle.

According to the invention, the particles added are ceramic particles. The man skilled in the art knows that technical or industrial ceramics refer to a class of manufactured materials that are non-metallic and inorganic. They are divided into two main groups: the oxides (for example Al₂O₃, TiO₂, SiO₂, MgO, ZrO₂, Y₂O₃, etc.) and the non-oxides (nitrides, carbides, borides, silicides, etc.). Moreover, for the requirements of the invention, the ceramic particles must comply with the following operational definition: they are of a nanometric size, typically 10-100 nanometres (1 nm=10⁻⁹ m), and after incorporation into the molten steel, they are essentially homogeneously distributed throughout the entire section of the cast product. The “size” of the particles is meant here as the largest dimension of the particle. The nanometric nature of the particles for inclusion is in fact indispensable to the reinforcement of the product. By contrast, micrometric inclusions constitute defects, heterogeneous areas that make the product weaker.

The quantities of nanoparticles added to the molten steel are maximum 1% by weight.

The wettability of the particles in the molten steel is the most important criterion for the choice of particles and the resolution of this technical problem is at the heart of the present invention. Homogeneous distribution of the nanoparticles in the molten steel is indispensable, which excludes confinement of the powders injected to the surface of the molten steel.

According to the invention, the particles may advantageously be conglomerated up to a size of 100-200 μm so as to be able to be injected through the HJN nozzle.

To improve the wettability of the particles in the molten steel, the nanometric ceramic particles may be conglomerated in an iron or metal matrix to obtain a compound whose characteristic final size is 100-200 μm. The iron or metal matrix favours the dispersion of the particles in the molten steel. In order to obtain this compound, nanometric ceramic particles are used mixed with micrometric iron particles (whose size is for example 10 to 20 microns). The mixture is produced either by:

-   -   mixing into a slurry and then drying, crushing, isostatic         pressing and then re-crushing;     -   high-energy tapping (mechanical alloying) to ensure that the         ceramics are incorporated into the iron matrix.         Tapping is an operation that consists in bringing an element         into contact and introducing it into a combination formed of one         or several elements that are different from the first element by         exerting a force on the element.

Advantageously, these compounds are injected under gaseous atmosphere in the HJN nozzle (see patent EP-B-605 379). The heavy turbulence occurring in the nozzle thus allows good incorporation of the particles into the molten steel. 

1-18. (canceled)
 19. Method for the continuous casting of metal in the form of a hollow jet in a nozzle positioned between a ladle or a tundish and a continuous casting ingot mould, said nozzle comprising in its upper part a distribution device capable of diverting at least part of the molten metal arriving at the inlet of the nozzle towards an inner wall of the nozzle before it enters the ingot mould, said method comprises the injection into an internal volume of the hollow jet of finely divided solid material comprising nanoparticles of technical ceramic with a characteristic size lower than 200 nm, said nanoparticles being conglomerated prior to their injection into the nozzle into microparticles of a size between 10 and 1,000 microns, characterised in that said nanoparticles are conglomerated into microparticles in a metal matrix.
 20. Method according to claim 19, characterised in that the characteristic size of the nanoparticles is lower than 100 nm.
 21. Method according to claim 20, characterised in that the size of the nanoparticles is between 10 and 100 nm.
 22. Method according to claim 19, characterised in that the size of the microparticles is between 100 and 200 microns.
 23. Method according to claim 19, characterised in that the metal matrix is made of the same metal as the cast metal.
 24. Method according to claim 19, characterised in that the nanoparticles of technical ceramic comprise nanoparticles of oxides, nitrides, carbides, borides, silicides and/or compounds thereof.
 25. Method according to claim 24, characterised in that the oxides are Al₂O₃, TiO₂, SiO₂, MgO, ZrO₂ or Y₂O₃.
 26. Method according to claim 19, characterised in that the quantity of nanoparticles incorporated into the molten metal is between 0.1 and 1% by weight of the cast metal.
 27. Method according to claim 19, characterised in that the conglomerated ceramic nanoparticles injected into the inner volume of the hollow jet of the nozzle are in suspension in a non-oxidising gas, said gas being at a slightly higher pressure relative to atmospheric pressure and at most equal to the static pressure of the cast metal upon its entry into the ingot mould.
 28. Method according to claim 19, characterised in that the conglomerated ceramic nanoparticles are injected into the inner volume of the hollow jet of the nozzle by means of a mechanical conveyance device.
 29. Method according to claim 19, characterised in that the cast metal is molten steel and the metal matrix is an iron matrix.
 30. Method according to claim 29, characterised in that the metal matrix comprises an alloy metal other than iron.
 31. Method according to claim 29, characterised in that the conglomeration of the nanoparticles is obtained by mixing ceramic nanoparticles with micrometric iron particles, i.e. with a size greater than 10 microns.
 32. Method according to claim 31, characterised in that said micrometric iron particles have a size lower than 20 microns.
 33. Method according to claim 31, characterised in that said mixture is produced by a premix in a slurry, followed by drying, crushing, isostatic pressing and re-crushing.
 34. Method according to claim 31, characterised in that said mixture is produced by high-energy tapping to ensure that the ceramics are incorporated into the iron matrix.
 35. Method according to claim 19, characterised in that the hollow jet nozzle used is of the rotary jet type, i.e. it comprises a vertical conduit having a distribution device with a dome in its upper part, whose function is to divert the molten metal entering the nozzle towards the inner surface of said conduit and which comprises a series of arms symmetrically arranged in a star pattern relative to the axis of the nozzle and canted relative to the horizontal, said arms being arranged to impart a helicoidal rotary motion to the molten steel along the internal wall of the nozzle.
 36. Method according to claim 19, characterised in that the hollow jet nozzle used comprises in its upper part a distribution device with a dome designed to separate the molten metal into two streams, an inner stream and an outer stream, in two physically well-separated zones, the injection of ceramic nanoparticles under the dome in the inner zone allowing the formation of a metal with a different chemical composition to that of the basic metal, cast in the outer zone.
 37. Method according to claim 36, characterised in that the injection of ceramic nanoparticles is alternatively produced in the outer zone.
 38. Metal with great mechanical strength having the form after casting of a ingot in a continuous sheet upon exit from a continuous casting ingot mould, which may be obtained by means of the method according to claim 1, comprising less than one percent by weight of technical ceramic homogeneously distributed in at least one part of the ingot. 